1. Field of the Invention
This invention relates to a DC motor control device and a DC motor control method especially for use as a countermeasure against heat generated at a carriage of a serial printer driven by a DC motor.
2. Related Background Art
First explained are general configuration of an ink jet printer using a DC motor control device and its control method.
FIG. 1 is a block diagram that shows general configuration of an ink jet printer.
The ink jet printer shown in FIG. 1 includes a paper feed motor (hereinafter also called a PF motor) 1 that feeds paper; a paper feed motor driver 2 that drives the paper feed motor 1; a carriage 3 that supports a head 9 fixed thereto to supply ink onto printing paper 50 and is driven to move in parallel to the printing paper 50 and vertically of the paper feeding direction; a carriage motor (hereinafter also called a CR motor) 4 that drives the carriage 3; a CR motor driver 5 that drives the carriage motor 4; a DC unit 6 that outputs a D.C. current for controlling the CR motor driver 5; a pump motor 7 that controls the draft of ink for the purpose of preventing clogging of the head 9; a pump motor driver 8 that drives the pump motor 7; a head driver 10 that drives and controls the head 9; a linear encoder 11 fixed to the carriage 3; a linear encoder coding plate 12 having slits in predetermined intervals; a rotary encoder 13 for the PF motor 1; a paper detecting sensor 15 that detects the terminal position of each sheet of paper under printing; a CPU 16 that controls the whole printer; a timer IC 17 that periodically generates interruption signals to the CPU 16; an interface portion (hereinafter also called IF) 19 that exchanges data with a host computer 18; an ASIC 20 that controls the character resolution, driving waveform of the head 9, and so on, in accordance with character information sent from the host computer 18 through the IF 19; a PROM 21, a RAM 22 and an EEPROM 23 that are used as an operation area of the ASIC 20 and the CPU 16 and a program storage area; a platen 25 that supports the printing paper 50; a transport roller 27 driven by the PF motor 1 to transport the printing paper 50; a pulley 30 attached to a rotating shaft of the CR motor 4; and a timing belt 31 driven by the pulley 30.
The DC unit 6 controls and drives the paper feed motor driver 2 and the CR motor driver 5 in response to a control instruction sent from the CPU 16 and outputs of the encoders 11, 13. Both the paper feed motor 1 and the CR motor 4 are DC motors.
FIG. 2 is a perspective view that illustrates configuration around the carriage 3 of the ink jet printer.
As shown in FIG. 2, the carriage 3 is connected to the carriage motor 4 by the timing belt 31 via the pulley 30, and driven to move in parallel with the platen 25 under guidance of a guide member 32. The carriage 3 has the recording head 9 projecting from its surface opposed to the printing paper and having a row of nozzles for releasing black ink and a row of nozzles for releasing color ink. These nozzles are supplied with ink from the ink cartridge 34 and release drops of ink onto the printing paper to print characters and images.
In a non-print area of the carriage 3, there is provided a capping device 35 for shutting nozzle openings of the recording head 9 when printing is not executed, and a pump unit 36 having the pump motor 7 shown in FIG. 1. When the carriage 3 moves from the print area to the non-print area, it contacts a lever, not shown, and the capping device 35 moves upward to close the head 9.
When any of the nozzle openings of the head 9 is clogged, or ink is forcibly released from the head 9 just after replacement of the cartridge 34, the pump unit 36 is activated while closing the head 9, and a negative pressure from the pump unit 36 is used to suck out ink from the nozzle openings. As a result, dust and paper powder are washed out from around the nozzle openings, and bubbles in the head 9, if any, are discharged together with the ink to the cap 37.
FIG. 3 is a diagram schematically illustrating configuration of the linear encoder 11 attached to the carriage 3.
The encoder 11 shown in FIG. 3 includes a light emitting diode 11a, collimator lens 11b and detector/processor 11c. The detector/processor 11c has a plurality of (four) photo diodes 11d, signal processing circuit 11e, and two comparators 11.sub.fA, 11.sub.fB.
When a voltage V.sub.CC is applied across opposite ends of the light emitting diode 11a through a resistor, light is emitted from the light emitting diode 11a. This light is collimated into parallel beams by the collimator lens 11b, and the beams pass through the coding plate 12. The coding plate 12 has slits in predetermined intervals (for example, in intervals of 1/180 inch).
Parallel beams passing through the coding plate 12 enter into photo diodes lid through fixed slits, not shown, and are converted into electric signals. Electric signals output from these four photo diodes 11d are processes in the signal processing circuit 11e. Signals output from the signal processing circuit 11e are compared in the comparators 11.sub.fA, 11.sub.fB, and comparison results are output as pulses. Pulses ENC-A, ENC-B output from the comparators 11.sub.fA, 11.sub.fB are outputs of the encoder 11.
FIGS. 4A and 4B are timing charts showing waveforms of two output signals from the encoder 11 during normal rotation of the CR motor and during its reverse rotation.
As shown in FIGS. 4A and 4B, in both normal rotation and reverse rotation of the CR motor, the pulse ENC-A and the pulse ENC-B are different in phase by 90 degrees. The encoder 4 is so configured that the pulse ENC-A is forward in phase by 90 degrees relative to the pulse ENC-B as shown in FIG. 4A when the CR motor 4 rotates in the normal direction, i.e., when the carriage 3 is moving in its main scanning direction whereas the pulse ENC-A is behind in phase by 90 degrees relative to the pulse ENC-B as shown in FIG. 4B when the CR motor 4 rotates in the reverse direction. Then, one period T of these pulses corresponds to each interval of the slits of the coding plate 12 (for example, 1/180 inch), and it is equal to the time required for the carriage 3 to move from a slit to another.
On the other hand, the rotary encoder 13 for the PF motor 1 has the same configuration as the linear encoder 11 except that the former is a rotatable disc that rotates in response to rotation of the PF motor 1, and the rotary encoder 13 also outputs two output pulses ENC-A, ENC-B. In ink jet printers, in general, slit interval of a plurality of slits provided on a coding plate of the encoder 13 for the PF motor 1 is 1/180 inch, and paper is fed by 1/1440 inch when the PF motor rotates by each slit interval.
FIG. 5 is a perspective view showing a part related to paper feeding and paper detection.
With reference to FIG. 5, explanation is made about the position of the paper detecting sensor 15 shown in FIG. 1. In FIG. 5, a sheet of printing paper 50 inserted into a paper feed inlet 61 of a printer 60 is conveyed into the printer 60 by a paper feed roller 64 driven by a paper feed motor 63. The forward end of the printing paper 50 conveyed into the printer 60 is detected by an optical paper detecting sensor 15, for example. The paper 50 whose forward end is detected by the paper detecting sensor 15 is transported by a paper feed roller 65 driven by the PF motor 1 and a free roller 66.
Subsequently, ink is released from the recording head (not shown) fixed to the carriage 3 which moves along the carriage guide member 32 to print something on the printing paper 50. When the paper is transported to a predetermined position, the terminal end of the printing paper 50 currently under printing is detected by the paper detecting sensor 15. The printing paper 50 after printing is discharged outside from a paper outlet 62 by a discharge roller 68 driven by a gear 67C, which is driven by the PF motor 1 via gears 67A, 67B, and a free roller 69.
Next explained are configuration of the DC unit 6, which is a conventional DC motor control device for controlling the CR motor 4 of the above-explained ink jet printer, and a control method by the DC unit 6.
FIG. 6 is a block diagram showing configuration of the DC unit 6 as the conventional DC motor control device. FIG. 7 is a graph that shows motor current and motor speed of the CR motor 4 controlled by the DC unit 6.
The DC unit 6 shown in FIG. 6 includes a position operator 6a, a subtracter 6b, a target speed operator 6c, a speed operator 6d, a subtracter 6e, a proportional element 6f, an integral element 6g, a differential element 6h, an adder 6i, a D/A converter 6j, a timer 6k, and an acceleration controller 6m.
The position operator 6a detects rising edges and tail edges of the output pulses ENC-A and ENC-B of the encoder 11, then counts the number of edges detected, and operates the position of the carriage 3 from the counted value. This counting adds "+1" when one edge is detected while the CR motor 4 rotates in the normal direction, and adds "-1" when one edge is detected while the CR motor 4 rotates in the reverse direction. Period of pulses ENC-A and period of pulses ENC-B are equal to the slit interval of the coding plate 12, and the pulses ENC-A and ENC-B are different in phase by 90 degrees. Therefore, the count value "1" of that counting corresponds to 1/4 of the slit interval of the coding plate 12. As a result, distance of the movement from the position of the carriage 3, at which the count value corresponds to "0", can be obtained by multiplying the above count value by 1/4 of the slit interval. Resolution of the encoder 11 in this condition is 1/4 of the slit interval of the coding plate 12. If the slit interval is 1/180 inch, then the resolution is 1/720 inch.
The subtracter 6b operates positional difference between the target position sent from the CPU 16 and the actual position of the carriage 3 obtained by the position operator 6a.
The target speed operator 6c operates the target speed of the carriage 3 from the positional difference output from the subtracter 6b. This operation is conducted by multiplying the positional difference by a gain K.sub.p. This gain K.sub.p is determined in accordance with the positional difference, and the value of the gain K.sub.p is stored in a table, not shown. This table is located in the PROM 21 or the EEPROM 23 shown in FIG. 1, for example, and the gain K, is sent through the CPU 16.
The speed operator 6d operates the speed of the carriage 3 on the basis of output pulses ENC-A, ENC-B of the encoder 11. This speed is obtained in the following manner. First, rising edges and tail edges of output pulses ENC-A, ENC-B of the encoder 11 are detected, and the duration of time between edges corresponding to 1/4 of the slit interval of the coding plate 12 is counted by a timer counter, for example. When the count value is T and the slit interval of the coding plate 12 is .lambda., the speed of the carriage is obtained as .lambda./(4T). Note here that operation of the speed is performed by measuring one period of output pulses ENC-A, e.g., from a rising edge to the next rising edge, by means of a timer counter.
The subtracter 6e operates speed difference between the target speed and the actual speed of the carriage 3 operated by the speed operator 6d.
The proportional element 6f multiplies the speed difference by a constant G.sub.p, and outputs its multiplication result. The integral element 6g cumulates products of speed differences and a constant Gi. The differential element 6h multiplies the difference between the current speed difference and its preceding speed difference by a constant Gd, and outputs its multiplication result. Operations of the proportional element 6f, the integral element 6g and the differential element 6h are conducted in every period of output pulses ENC-A of the encoder 11, synchronizing with the rising edge of each output pulse ENC-A, for example.
Outputs of the proportional element 6f, the integral element 6g and the differential element 6h are added in the adder 6i. Then, the result of the addition, i.e., the drive current of the CR motor 4, is sent to the D/A converter 6j and converted into an analog current. Based on this analog current, the CR motor 4 is driven by the driver 5.
The timer 6k and the acceleration controller 6m are used for controlling acceleration whereas PID control using the proportional element 6f, the integral element 6g and the differential element 6h is used for constant speed and deceleration control during acceleration.
The timer 6k generates a timer interrupt signal every predetermined interval in response to a clock signal sent from the CPU 16.
The acceleration controller 6m cumulates a predetermined current value (for example 20 mA) to the target current value every time it receives the timer interrupt signal, and results of the integration, i.e, target current values of the DC motor during acceleration, are sent to the D/A converter 6j from time to time. Similarly to PID control, the target current value is converted into an analog current by the D/A converter 6j, and the CR motor 4 is driven by the driver 5 according to this analog current.
The driver 5 has four transistors, for example, and it can create (a) a drive mode for rotating the CR motor 4 in the normal or reverse direction; (b) a regeneration brake drive mode (a short brake drive mode, which is the mode maintaining a halt of the CR motor); and (c) a mode for stopping the CR motor, by turning those transistors ON or OFF in accordance with outputs from the D/A converter 6j. Further, the driver 5 is so configured that, in the drive mode for rotating the CR motor in the normal or reverse direction, it can supply a desired current to the CR motor 4 by changing intensities of signals applied to gates of those transistors.
Next explained is the performance of the DC unit 6, that is, the conventional DC motor control method, with reference to FIGS. 7A and 7B.
While the CR motor 4 stops, when a start instruction signal for starting the CR motor 4 is sent from the CPU 16 to the DC unit 6, a start initial current value I.sub.0 is sent from the acceleration controller 6m to the D/A converter 6j. This start initial current value I.sub.0 is sent together with the start instruction signal from the CPU 16 to the acceleration controller 6m. Then, this current value I.sub.0 is converted into an analog current by the D/A converter 6j and sent to the driver 5 which in turn start the CR motor 4 (see FIGS. 7A and 7B). After the start instruction signal is received, the timer interrupt signal is generated every predetermined interval from the timer 6k. The acceleration controller 6m cumulates a predetermined current value (for example, 20 mA) to the start initial current value I.sub.0 every time it receives the timer interrupt signal, and sends the cumulated current value to the D/A converter 6j. Then, the cumulated current value is converted into an analog current by the D/A converter 6j and sent to the driver 5. Then, the CR motor is driven by the driver 5 so that the value of the current supplied to the CR motor 4 becomes the cumulated current value mentioned above, and the speed of the CR motor 4 increases (see FIG. 7B). Therefore, the current value supplied to the CR motor 4 represents a step-like aspect as shown in FIG. 7A. At that time, the PID control system also works, but the D/A converter 6j selects and employs the output from the acceleration controller 6m.
Cumulative processing of current values of the acceleration controller 6m is continued until the cumulated current value reaches a fixed current value I.sub.s. When the cumulated current value reaches the predetermined value I.sub.s at time t1, the acceleration controller 6m stops its cumulative processing, and supplies the fixed current value I.sub.s to the D/A converter 6j. As a result, the CR motor 4 is driven by the driver 5 such that the value of the current supplied to the CR motor 4 becomes the current value I.sub.s (see FIG. 7A).
In order to prevent the speed of the CR motor 4 from overshooting, if the speed of the CR motor 4 increases to a predetermined value V1 (see time t2), the acceleration controller 6m makes a control to reduce the current supplied to the CR motor 4. At that time, the speed of the CR motor 4 further increases, but when it reaches a predetermined speed Vc (see time t3 of FIG. 7B), the D/A converter 6j selects the output of the PID control system, i.e., the output of the adder 6i, and PID control is effected.
That is, based on the positional difference between the target position and the actual position obtained from the output of the encoder 11, the target speed is operated, and based on the speed difference between this target speed and the actual speed obtained from the output of the encoder 1, the proportional element 6f, the integral element 6g and the differential element 6h act to perform proportional, the integral and the differential operations, respectively, and based on the sum of results of these operations, the CR motor 4 is controlled. These proportional, integral and differential operations are conducted synchronously with the rising edge of the output pulse ENC-A of the encoder 11, for example. As a result, speed of the DC motor 4 is controlled to be a desired speed Ve. The predetermined speed Vc is preferably a value corresponding to 70 through 80% of the desired speed Ve.
From time t4, the DC motor 4 reaches the desired speed, and the carriage 3 also reaches the desired constant speed Ve and can perform printing.
When the printing is completed and the carriage 3 comes close to the target position (see time t5 in FIG. 7B), the positional difference becomes smaller, and the target speed also becomes slower. Therefore, the speed difference, i.e., the output of the subtracter 6e becomes a negative value, and the DC motor 4 is decelerated and stops at time t6.
However, the control of the DC motor by the above-reviewed conventional DC motor control device and control method involved the problem that thermal resistance increased by heat generation of the DC motor caused by its continuous motion, and disturbed a sufficient flow of electric current into the DC motor. Insufficient flow of the current into the DC motor resulted in degrading the control accuracy of the CR motor that was a DC motor, and deteriorating the printing performance of the ink jet printer.
As its countermeasures, braking during the driving motion or limitation of the continuous printing length, for example, was employed conventionally. However, there remained the problem that the whole driving motion time largely increased.